I understand the ABO and Rhesus systems quite well but I don't understand why O is the universal donor if O contains anti-A and anti-B antibodies. Wouldn't it agglutinate A, B and AB type blood? Or is the agglutination negligible? Or are antibodies removed from blood destined for transfusion?

O lacks the A and B surface antigens, therefore the receiver's immune system does not destroy thehse blood cells. O blood does have A and B antibodies, but since the volume transfused is usually relatively small, the little amounts of antibody that are transfered to the recipient are not a big problem. However, as far as I have understood, in big operations or when there has been a massive loss of blood, matching type of blood is always the optimal choice, and only if it is not possible, O type is given.

Infarious wrote:I understand the ABO and Rhesus systems quite well but I don't understand why O is the universal donor if O contains anti-A and anti-B antibodies. Wouldn't it agglutinate A, B and AB type blood? Or is the agglutination negligible? Or are antibodies removed from blood destined for transfusion?

the anti-A and anti-B antibodies are found in plasma...

plasma is separated from red blood cells by centrifugation...

only the O red blood cells are transfused (not the plasma)...

in clinical practice you'd always match blood types as it is not possible to fully separate plasma and red blood cells...

It is indeed so that the antibodies are found in the plasma (and on the surface of B-cells in the form of a membrane-bound immunoglobulin), but since both are separated from the red blood cells, it is not a big problem. Tiny amounts of both plasma and white blood cells (including B lymphocytes) usually remain among the erythrocytes, but like I tried to say, the amount is so small that usually it causes no harm. Despite this, in big transfusions this can have an effect, if O blood is given to a non-O recipient.

As far as I know, the antibodies remaining within the erythrocytes need not be removed, and I'm not even sure if there is an efficient / cost effective way of doing that. After all, virtually all antibodies are removed when the plasma is separated. Any antibody traces remaining with the red blood cells should not be a problem. At least during some smallish operation requiring a unit or two of red blood cells, whatever antibodies there are with them is not a problem. And like I said before, in bigger operations a matching type of blood is used if available.

Want to specify a bit what you meant with this 'need to be removed'? :)